A MILO consists of an electron-emitting cathode with an adjacent slow wave structure in a configuration similar to a linear magnetron. However, unlike a linear magnetron, there is no external means for producing a magnetic field in the space between the cathode and the adjacent slow wave structure. The insulating magnetic field is generated by current flow through the device itself. Such a device is illustrated in FIG. 1 which has cylindrical geometry so that the cathode is coaxial with the slow wave structure. The load at the output end of the device is in the form of a diode gap.
In use, a pulsed high potential is provided between the cathode and the slow wave structure. As a result, electrons are emitted from the cathode and are accelerated by the radial electric field. If this field is sufficiently large, magnetically insulated flow becomes established, where current flow at the diode region maintains an azimuthal magnetic field in the interaction space between the cathode and the slow wave structure. The combined effect of the radial electric field and the azimuthal magnetic field is to cause electrons emitted from the cathode to be confined in the region of the cathode and move axially to the output end of the device interacting with the slow wave structure as they do so in a manner analogous to that in a linear magnetron to produce microwave energy which is extracted from the output end of the slow wave structure.
A MILO with three or more cavities oscillates readily in its fundamental .pi.-mode. In this mode, each cavity in the slow wave structure has quarter wave oscillations shifted in phase by approximately .pi. from its neighbour. The quarter wave oscillations have maximum magnetic field at the cavity top, and maximum electric field close to the electron flow. As in the magnetron, the crossed field electron flow in the MILO develops a spoke-like structure as the electrons give up their potential and kinetic energy to the electromagnetic field.
Although large amplitude oscillations in the .pi.-mode are readily obtained, extracting power from these oscillations is not straightforward. The reason for this, which has been known for some time, is that close to the .pi.-mode the group velocity is small, so power cannot be transported rapidly out of the oscillator.
Possible solutions which have been considered are multicavity extraction and operation in .pi./2-mode. Multicavity extraction presents problems in the collection of power from multiple extraction ports. Operation in .pi./2-mode has been achieved by a MILO in which the slow wave structure has an input section which operates in .pi.-mode and modulates the electron flow. The output section is designed to have a natural .pi.-mode at twice the frequency of the input .pi.-mode but is driven in the .pi./2-mode by the input section. The problem with this approach is that the output section tends to self-oscillate in its own .pi.-mode with consequent loss of power output.